1. Field of the Invention
The invention relates to image scanning, and in particular the design and implementation of scanning devices to allow for the scanning of documents with fragile front surfaces, specifically by providing for positioning a document at the focus of the scanning device without sliding motion occurring across the front surface of the document.
2. Description of the Problem and Prior Art
High-quality "scanning" of the image contents of physical documents, such as business documents, into digital form is routinely accomplished in commercially available devices such as computer-attached document scanners and facsimile machines. Document scanners are used, for example, to convert images to digital form for inclusion in computer files. Facsimile machines transmit the digital information over telephone lines to other facsimile machines, which then recreate a facsimile of the original document. The conversion of the document from physical to digital form is accomplished in an image sensor unit. Traditionally, the image sensor is a charge coupled device (CCD) or a silicon image sensor. Illumination is provided by an illuminating device, while the optics for focusing the surface of the document to the image sensor is provided with one or more mirrors and lenses. This optical arrangement allows both for the possibility of document reduction or enlargement and for control over the depth of field. The depth of field of such an arrangement can be large, making scanning without the necessity of making contact (herein "non-contact scanning" practical with such an optical arrangement.
Recently, a compact integrated device, called a contact image sensor (CIS) has started to be used for the image sensor unit. A CIS typically incorporates illumination, focusing optics in the form of a narrow depth of focus lens array, for example a gradient index lens array (also called a self-focussing lens array), and one or more sensor arrays in one package. The compact unit is often sealed, with illumination and document scanning occurring through a single "scanner window. " Scanning devices are constructed so that the CIS image sensor unit scans the width of the document, which is placed at the focus of the unit, while the length of the document is scanned through induced relative motion between the document and the sensor unit. The advantages of a CIS over traditional sensor technology include: ease of manufacturing, reduced component count, fewer moving parts, reduced cost, integrated optics, and environmentally controlled optical path. One of the disadvantages is a rather narrow depth of focus, as described further below.
Relative motion between the document and the image sensor unit is obtained through one of two configurations. The first typically is used by "flat bed" scanners and copiers. In such a configuration, the CIS usually does not have a physical scanner window, and the document is placed on top of a sheet of transparent material with an image scanner located below and focused through the transparent material. The CIS image scanner unit is forced to move the length of the document through a drive mechanism while the document remains stationary. This configuration is not the main area of application of the present invention.
A second configuration is suitable for use in facsimile machines, hand-held scanners and sheet-fed scanners, and is the main area of application of the present invention. In this configuration the CIS unit is arranged so that the focus of the sensor-lens system is coincident with the outer surface of a window that is substantially fixed to the CIS. The document must be placed substantially in contact with the scanner window for the document to be in focus, and document-sensor window contact must be maintained while scanning in order to keep the document in the shallow focal region. Because of the utility of this arrangement, sealed contact image sensors are common in facsimile machines and other inexpensive document scanners, and devices that use them must be configured so that the document to be scanned slides across the scanner window.
Because of the advantages of the contact image sensor arrangement such as relative durability and low cost, there is near ubiquity of facsimile machines and low cost of black and white or gray scale document scanners using this technology. Contact image sensors usually have a very small depth of field however, on the order of two tenths of a millimeter, and so high-quality scanning is accomplished only with a document in good contact with the scanner window. The relatively narrow depth of field of contact image sensors results in design problems that are not present in image sensors where the depth of field can be varied. The recent introduction of color contact image sensors provides yet another design problem: color scanners are used for photographs as well as printed documents, and thus the contact image sensor must be compatible with photographic media. Contact is not a problem with many types of common document material, such as smooth paper. However, scanners based on contact image sensors technology suffer from the inability to scan materials that cannot pass smoothly by the scanner unit. Specifically, photographic documents cannot reliably be used with contact image sensors. This inability manifests itself in the following ways:
1. Due to the soft, water absorbing properties of photographic emulsions, "wetted" contact between the photographic document and the scanner window may occur. When this happens, the coefficient of friction between the document and scanner window is increased, the document sticks to the scanner window, and document transport problems result.
2. The optical properties of the document/scanner window interface vary between wetted and non-wetted contact points. The partial sticking of the photographic document to the window results in streaks or spots on the scanned image which is denoted here as "spotting."
3. Soft photographic emulsions are much more susceptible to damage than are paper documents. Photographic documents may be harmed or scratched as a result of either the direct contact of the sliding contact with the window, or by dirt particles that may come between the document and the scanner window.
The inability to use this type of scanning technology on photographs is acknowledged by manufacturers of scanners, some of who issue protective photograph covers of clear plastic for use when scanning photographs. Thus there is a need in the art for overcoming the problems encountered with scanning by CISs of documents, such as photographs, that may stick to the scanner window or be damaged by contact.